Polysiloxanes, especially polydimethylsiloxanes have long been used in applications where lubricity, hydrophobicity, low free-surface energy, and good low temperature flexibility, biocompatibility and/or oxygen permeability are of great concern. Aside from the specific synthesis technique for making silicone rubber, most of the general techniques of polymerization have also been used to make silicone-containing polymers and especially block-copolymers with a desirable combination of properties. These techniques include free radical polymerization of oligosiloxanylalkyl (meth)acrylates and, more commonly, synthesis of step-growth polymers like polyesters, polyurethanes or polyamides. Useful polysiloxane intermediates for this purpose are polydimethylsiloxanes of either linear or branched structure which contain two or more reactive groups, such as hydroxyl-, amino-, or thiol groups, attached by an intervening alkylene group to a Si-atom. From these intermediates vinyl terminated prepolymers have been prepared for instance by direct esterification or amidification with acryloyl- or methacryloyl chloride, acrylic- or methacrylic acid or by trans-esterification with the corresponding lower alkyl esters.
These reactions, although not difficult to carry out, are not practical when a precise end product is desired, since high conversions are difficult to achieve, requiring excess reactants as well as prolonged heating for the removal of HCl, water or alcohols. Another approach, avoiding these disadvantages involves reaction of the above mentioned polysiloxane intermediates with an equivalent amount of diisocyanate, followed by endcapping with a hydroxy- or aminoalkyl (meth)acrylate, as for instance described in U.S. Pat. No. 4,136,250, U.S. Pat. No. 4,130,708 and U.S. Pat. No. 4,486,577. Since urethane and urea formation proceeds at low temperatures and without evolution of by-products, this is a far more practical synthesis, especially where the final objective is a contact lens with uniform properties and maximal reproducibility. Further advantages of polyurethanes and polyureas over polyesters are their greater strength and flexibility and better compatibility and clarity in combination with second polymers, this being a result of inter-chain hydrogen bonding as in nylon.
However, a remaining disadvantage of the structure and synthesis described in U.S. Pat. Nos. 4,136,250, and 4,486,577, is, that due to the statistical nature of step-growth polymerization kinetics a certain amount of chain extension reactions always occurs during the reaction of the diisocyanate with the polysiloxane-diol, leading in the best case to a difficult to control or reproduce increase in viscosity and increase in poly-dispersity of the prepolymer, and in the worst case to premature gelation or at least the danger of premature gelation. Although in applications such as photo-curable coatings, small variations in polymer dispersity and functionality are of little concern, in other applications, for instance in the preparation of oxygen permeable hard and soft contact lenses precise batch-to-batch reproducibility is of utmost importance in order to meet narrow specifications of hardness, flexibility and clarity.
It has now been discovered that the above mentioned problems can be overcome and a radiation or thermally curable superior unsaturated polydimethylsiloxane be synthesized if a polydimethylsiloxane of 500-10,000 molecular weight, and containing at least two --OH, --NH or --SH groups is reacted with 2-isocyanatoethyl methacrylate (IEM), 2-isocyanoethyl acrylate, 3-isocyanatopropyl methacrylate, 1-methyl- and 1,1-dimethyl-2-isocyanatoethyl acrylate or methacrylate, of which 2-isocyanatoethyl methacrylate is preferred. Also useful in the context of the present invention are vinyl isocyanate and di(2-isocyanatoethyl) fumarate.
Reaction products of a wide variety of polyols with 2-isocyanatoethyl methacrylate have been described, for instance to be used as radiation curable coatings (U.S. Pat. No. 4,233,425), as an aerobic adhesive (U.S. Pat. No. 4,320,221) or as clear, tack-free fluorinated protective coatings (GB Pat. No. 2,050,396 B). In no instance have functional polysiloxanes been used as in the present invention.